Merge remote-tracking branch 'remotes/dgilbert-gitlab/tags/pull-migration-20210726a...

Merge remote-tracking branch 'remotes/dgilbert-gitlab/tags/pull-migration-20210726a' into staging

Migration fixes 2021-07-26

Peter's fix for a bunch of races
 -> Seem to fix the occasional crash seen by Peter

Wei's fix for migration with free page hinting
 -> Bug has been around for a while, but makes a huge difference

My fix for OpenBSD test corner case

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
# gpg: Signature made Mon 26 Jul 2021 13:42:16 BST
# gpg:                using RSA key 45F5C71B4A0CB7FB977A9FA90516331EBC5BFDE7
# gpg: Good signature from "Dr. David Alan Gilbert (RH2) <dgilbert@redhat.com>" [full]
# Primary key fingerprint: 45F5 C71B 4A0C B7FB 977A  9FA9 0516 331E BC5B FDE7

* remotes/dgilbert-gitlab/tags/pull-migration-20210726a:
  migration: clear the memory region dirty bitmap when skipping free pages
  migration: Move the yank unregister of channel_close out
  migration: Teach QEMUFile to be QIOChannel-aware
  migration: Introduce migration_ioc_[un]register_yank()
  migration: Make from_dst_file accesses thread-safe
  migration: Fix missing join() of rp_thread
  tests/qtest/migration-test.c: use 127.0.0.1 instead of 0

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>